Origin of the in situ stress field in south-eastern Australia

The in situ stress field of south‐eastern Australia inferred from earthquake focal mechanisms and bore‐hole breakouts is unusual in that it is characterised by large obliquity between the maximum horizontal compressive stress orientation (S_Hmax) and the absolute plate motion azimuth. The evolution of the neotectonic strain field deduced from historical seismicity and both onshore and offshore faulting records is used to address the origin of this unusual stress field. Strain rates derived from estimates of the seismic moment release rate (up to ～10^?16 s^?1) are compatible with Quaternary fault–slip rates. The record of more or less continuous tectonic activity extends back to the terminal Miocene or early Pliocene (10–5 Ma). Terminal Miocene tectonic activity was characterised by regional‐scale tilting and local uplift and erosion, now best preserved by unconformities in offshore basins. Plate‐scale stress modelling suggests the in situ stress field reflects increased coupling of the Australian and Pacific Plate boundary in the late Miocene, associated with the formation of the Southern Alps in New Zealand.     

Relative contributions of silicate and carbonate rocks to riverine Sr fluxes in the headwaters of the Ganges

Exhumation of the Himalayan-Tibetan orogen is implicated in the marked rise in seawater ⁸⁷Sr/⁸⁶Sr ratios since 40 Ma. However both silicate and carbonate rocks in the Himalaya have elevated ⁸⁷Sr/⁸⁶Sr ratios and there is disagreement as to how much of the ⁸⁷Sr flux is derived from silicate weathering. Most previous studies have used element ratios from bedrock to constrain the proportions of silicate- and carbonate-derived Sr in river waters. Here we use arrays of water compositions sampled from the head waters of the Ganges in the Indian and Nepalese Himalaya to constrain the end-member element ratios. The compositions of tributaries draining catchments restricted to a limited range of geological units can be described by two-component mixing of silicate and carbonate-derived components and lie on a plane in multicomponent composition space. Key elemental ratios of the carbonate and silicate components are determined by the intersection of the tributary mixing plane with the planes Na = 0 for carbonate and constant Ca/Na for silicate. The fractions of Sr derived from silicate and carbonate sources are then calculated by mass-balance in Sr-Ca-Mg-Na composition space. Comparison of end-member compositions with bedrock implies that secondary calcite deposition may be important in some catchments and that dissolution of low-Mg trace calcite in silicate rocks may explain discrepancies in Sr-Ca-Na-Mg covariation. Alternatively, composition-dependent precipitation or incongruent dissolution reactions may rotate mixing trends on cation-ratio diagrams. However the calculations are not sensitive to transformations of the compositions by incongruent dissolution or precipitation processes provided that the transformed silicate and carbonate component vectors are constrained. Silicates are calculated to provide ∼50% of the dissolved Sr flux from the head waters of the Ganges assuming that discrepancies between Ca-Mg-Na covariation and the silicate rock compositions arise from addition of trace calcite. If the Ca-Mg-Na mixing plane is rotated by composition-dependent secondary calcite deposition, this estimate would be increased. Moreover, when ⁸⁷Sr/⁸⁶Sr ratios of the Sr inputs are considered, silicate Sr is responsible for 70 ± 16% (1σ) of the ⁸⁷Sr flux forcing changes in seawater Sr-isotopic composition. Since earlier studies predict that silicate weathering generates as little as 20% of the total Sr flux in Himalayan river systems, this study demonstrates that the significance of silicate weathering can be greatly underestimated if the processes that decouple the water cation ratios from those of the source rocks are not properly evaluated.     

Bar-top hollows: A new element in the architecture of sandy braided rivers

Discrete hollows in the bar tops of the South Saskatchewan River are described that form a newly-recognized morphological element of sandy braided rivers. These bar-top hollows, which are up to 1.7 m deep and may extend for 10–30 m down and across flow, have a circular to ovoid planform and are shown, through use of ground penetrating radar, to be filled by a series of distinct, often angle-of-repose, foresets. The hollows form by both erosion and bar-top deposition and may be generated by bar-tail accretion, cross-bar channel cutoff and subsequent fill or lateral accretion at the bar-head. Bar-top hollows occur in the upper part of the bar depositional sequence and may thus prove useful indicators for braid bar reconstruction in ancient sediments, and should not be confused with channel scour.     

用ANUDEM建立水文地貌关系正确DEM的方法研究

针对区域尺度的径流、水土流失定量评价和植被适宜性评价等研究工作需要,利用1∶25万数字地形图和ANUDEM软件,对黄土丘陵区中等分辨率水文地貌关系正确DEM建立方法进行了研究。结果表明该方法所建立的DEM,可以正确反映地貌梁、沟结构及其与流水线网络的关系,对地形描述的能力优于TIN方法建立的DEM;利用ANUDEM和1∶25万地形图插值建立黄土丘陵区DEM的三个主要参数分别为分辨率50或100,计算迭代次数40,第二糙率系数0.8。     

A Model to Predict Surface Temperatures

A model to predict the surface temperature of a variety of surfaces is described. The model solves the surface energy balance equation iteratively, using only standard meteorological data. Since surface and soil temperature information is not required for initialisation, the model is portable and, in theory, could be used for any surface and location. It is shown that, in order to obtain the correct cooling rates for vegetation during the night, the direct influence of the ground flux must be removed from the energy balance equation for the layer of vegetation. A scheme that couples a vegetation canopy to the ground solely by radiation is described, giving satisfactory cooling rates when compared with observations. Observations from a field site at Cardington, near Bedford, UK, are used to test the accuracy of the model for road and grass surfaces. When compared against these data, the model predicts surface temperatures with a root mean square error of about 1 °C for the road and 2 °C for the grass. Data from other sources not only give similar results to the Cardington data, but also demonstrate that the model can reproduce the characteristics of wet and partially dry soils and also dry desert sand. A study of the sensitivity of the model to errors in the forcing data indicates that inaccuracies in the air temperature data lead to similar sized errors in the predicted surface temperatures. Fluctuations in the forcing data that are not resolved by the model will affect a grass surface much more than a road surface, due to the relatively small thermal inertia of the grass.     

On the asphericity of nova remnants caused by rotating white dwarf envelopes

The effect of rotating white dwarf envelopes in determining the structure of nova shells is examined. This is achieved by numerical hydrodynamic simulations of the flows around a binary star system. In previous studies of remnant formation, this rotation has not been included.
It is found that the structures formed in the flow are more consistent with observations of nova shells than the previous theoretical studies. The shells produced by the nova become more prolate with increasing white dwarf envelope rotation. Hence the rotation of white dwarf envelopes must be included in any future discussion of remnant formation.
A possible method of identifying the dominant process by which mixing of accreted and white dwarf matter takes place is suggested.     

藏南深反射测线附近地表地质观察研究成果

本文简要介绍了ＩＮＤＥＰＴＨ项目地质组１９９４年和１９９５年间沿ＩＮＤＥＰＴＨ—Ｉ路线进行的野外填图工作的初步研究成果。它们是：①在帕里西北确定了高喜马拉雅与特提斯喜马拉雅之间拆离带的存在；③在帕里北发现了一套特提斯沉积，从而提出帕里西北的拆离带有可能穿过南北方向的亚东－谷露裂谷，而没有大规模错开；③在特提斯喜马拉雅带中确定了两条具一定规模的逆冲断裂；④发现康马穹窿存在两期构造变形，早期为上盘向北运动，晚期为横弯造穹作用。最后，结合深反射资料进一步讨论了康马穹窿与藏南拆离带之间的关系。